Chemistry of Post 12 Ma Los Frailes Volcanic Complex Ignimbrites in Bolivia and the Role of Magmatism in the Uplift of the Central Andean Altiplano Plateau

KAY, S.M., KELLER, C.B., COIRA, B., JIMÉNEZ, N., CAFFE, P.J.

San Francisco, California

Congreso; American Geophysical Union, Fall Meeting 2010; 2010

American Geophysical Union

The giant (~2000 km3) backarc Los Frailes volcanic complex in the Bolivian Altiplano of the Central Andes records the late Oligocene to Pleistocene magmatic history of the Altiplano plateau whose time and processes of uplift are controversial.  Most previous studies of the Los Frailes complex have concentrated on pre-12 Ma volcanic rocks that host Sn-Ag deposits including the world?s largest silver deposit. Here, we combine sparse pre-existing chemical data from post 12 Ma volcanic rocks with new analyses to show that the voluminous post-12 Ma ignimbrites are predominantly peraluminous, K-rich biotite-bearing andesites and dacites (58-69% SiO2) that can contain calcic feldspar, orthopyroxene, cordierite, and ilmenite. Trace element data show that they have very steep REE patterns (La/Yb=30-90) marked by HREE depletion (Sm/Yb=5-9) requiring garnet as a deep-crustal restitic phase.  The presence of cordierite requires final equilibration of the erupted magmas at less than ~14 km depth (< 450 MPa). Other prominent chemical features include LIL enrichment (to 90 ppm La), non-arc like Ba/La ratios (